I was peddling when peddle pressure dropped in half and I slowed down. I pulled the drive a found that one fin was coming lose from the drive. The post(rod) with notches was coming out. I pushed it back in. It seemed fixed but after a few minutes it came out again. Anyone have any suggestions?

Most likely one (or both) of two things: 1) your mast retaining screw has backed out enough to allow the mast to slip out or 2) your mast hole (part of your sprocket) has widened under use and needs to be replaced.